Add coverage reporting/badges and split CI between Gitea and GitHub (#5)

Merge dotnet+jest coverage via reportgenerator, publish a self-hosted
coverage badge and build-status badges on the readme. Gitea remains the
full pipeline (build/test/docker push/deploy-qa/smoke); GitHub only
builds and tests since it has no registry secrets or qa runner.

Reviewed-on: #5

.gitignore

@@ -476,3 +476,6 @@ ehthumbs.db
 
 # Self-installed .NET SDK (scripts/ci/lib.sh ensure_dotnet, used when a CI runner lacks the SDK)
 /.dotnet/
+
+# Self-installed reportgenerator CLI (scripts/ci/lib.sh ensure_reportgenerator)
+/.dotnet-tools/